The present invention relates to light sources, and more particularly, to a solid state light source for use in projectors and the like.
Systems for projecting images onto screens are known in the art. Systems based on incandescent or arc lamps are often used in projection displays. However, such sources have limited lifetimes and poor output stability over the life of the light source. In addition, spectral components, such as UV and IR, that are not needed for display purposes must be filtered out of the emissions to avoid damage to other components or the viewer""s eyes.
Sources based on combinations of Light Emitting Diodes (LEDs) or semiconductor lasers that emit red, green and blue light can be used as a solid state light source. Since LED and laser emissions are typically spectrally narrow band light sources, the use of color filters for the display can be omitted. Also the optical output of semiconductor lasers and LEDs can typically be electrically modulated at a rate that is sufficient for a time sequential color display. Furthermore most solid state lasers emit linear polarized light which further improves the efficiency of light production and simplifies the optics in projection systems.
Unfortunately, individual solid state light sources do not provide sufficient brightness for many projection display applications; hence, arrays of LEDs must be utilized to obtain sufficient output. However, construction of an array of LEDs with the correct color balance without losing brightness is difficult. In order to provide a full color display the solid state light source has to include emitters of at least three different colors. To maximize the brightness of a color display, the relative area covered by emitters of each color has to be adjusted according to the respective emitter radiance. If all emitters are driven simultaneously, the resulting color shining onto the light modulator used to generate the image must be white. In general, different color emitters will have different output powers per unit area. Hence, to provide maximum brightness, the array must accommodate different individual emitter dimensions. Furthermore the pattern of the individual solid state color emitters has to provide good mixing of the different color emitters in order to maximize the uniformity of illumination at the light valve of each individual color. These constraints are difficult to accommodate in a single two-dimensional source array.
This problem is complicated further by the limitations of present LED technology. Presently, LEDs have higher radiance in the red and blue spectral regions than in the green region of the spectrum. Hence, significantly different emitter areas are needed to provide a balanced output because of the poor radiance of the green detectors.
Broadly, it is the object of the present invention to provide an improved solid state light source for use in projection displays and the like.
It is a further object of the present invention to provide a light source that provides a white light output utilizing LEDs of different radiances.
It is yet another object of the present invention to provide a light source that has a higher radiance than a conventional two-dimensional array of LEDs.
These and other objects of the present invention will become apparent to those skilled in the art from the following detailed description of the invention and the accompanying drawings.
The present invention is a light source having emissions in a plurality of spectral bands. The simplest light source according to the present invention is constructed from first and second light sources and a dichroic beam splitter. The first and second solid state light sources emit light in first and second bands whose wavelengths do not completely overlap. The dichroic beam splitter combines light from the first and second solid state light sources by passing light in the first band while reflecting light in the second band to generate light having a spectral content in the first and second bands. The solid state light sources are preferably arrays of LEDs or VCSELs. A light source having improved radiance in the green region of the spectrum can be constructed by utilizing first and second solid state light sources that emit in different bands in the green portion of the spectrum. A white light source can be constructed by utilizing a third solid state light source and a second dichroic beam splitter and by choosing the emission bands of the light sources such that light leaving the second dichroic beam splitter is perceived as white by an observer when all three solid state light sources are emitting light.